In recent years, display devices such as liquid crystal displays, plasma displays, organic electroluminescent displays and electronic paper, input sensors such as touch panels, and solar cells utilizing sunlight such as thin film amorphous silicon solar cells and dye sensitized solar cells have been increasingly used. With the increase in use of these devices, demand for transparent conductive films, which are essential to these devices, has been increased as well.
Among others, films of ITO, a transparent oxide, or an oxide of In and Sn, are largely used. One process to produce films of ITO is application of minute particles dispersed in a solution to base films. However, vapor deposition with sputtering equipment or deposition equipment is normally employed to enhance transparency and to improve electric conductivity. On the other hand, the equipment for the deposition is massive and complicated, and the energy consumption by such equipment to produce the thin films is considerably large. Thus, the development of technologies to reduce the production cost and environmental burden has been desired. Furthermore, enlargement of the area of transparent conductive materials has been sought, which is accompanied by increasing demand for weight saving, flexibility, and lower resistance of transparent conductive materials.
Receiving the demand, researchers have been studying transparent electrically conductive films having nanowire of a metallic element, which can be produced by wet processes, and are excellent in lightness and flexibility. The nanowire of a metallic element has such a small diameter as 250 nm or less, which enables films having nanowires of a metallic element to have high optical transparency in the visible light region. Films having metallic nanowires are expected to have many applications as a transparent electric conductive film in place of films having ITO. Special attention is riveted to transparent conductive films having silver nanowires, which have high electric conductivity and stability (see, for example, patent document 1).
One process for producing silver nanowires comprises allowing a silver compound to react with polyvinylpyrrolidone, as an agent for controlling the growth of silver nanowires, in a polyol. Specifically, the following processes have been proposed: heating a silver compound with polyvinylpyrrolidone in a polyol (see, for example, patent document 2 and non-patent document 1), allowing a silver compound to react with polyvinylpyrrolidone and a quaternary ammonium chloride in a polyol (see, for example, patent document 3), allowing a silver compound to react with polyvinylpyrrolidone, iron ions, and chloride ions in a polyol (see, for example, patent document 4), and allowing a silver compound to react with polyvinylpyrrolidone, copper ions, and chloride ions (see, for example, non-patent document 2).
The transparent conductive film having silver nanowires shows electric conductivity through three-dimensional electrically conductive network structures formed by mutual contact of silver nanowires, the network structures spatially widely distributed in the film. Therefore silver nanowires whose length along their major axis has an optimal length should be employed depending on the use of the transparent conductive film. When silver nanowires are produced with polyvinylpyrrolidone as an agent for controlling the growth of silver nanowires, proposed is a process of controlling the length of silver nanowires along their major axis by adjusting the ratio of the mass of silver atoms to that of chloride ions (see, for example, patent document 5). However, the length along the major axis of the silver nanowires realized by this process is about 100 μm at most. Another proposal to address this problem is a process where concentrated nitric acid is used to lengthen the length of silver nanowires along their major axis (see, for example, patent document 4). This process, however, is capable of controlling the length of silver nanowires to approximately 300 μm at most. Besides, this process requires adding concentrated nitric acid at 130° C., which causes problems of reproducibility and safety. When silver nanowires are produced by a process using polyvinylpyrrolidone as an agent for controlling the growth of silver nanowires, it is difficult to widely control the length of silver nanowires along their major axis in the region where the length is not less than 100 μm.